Reading every nth frame from VideoCapture in OpenCV

Question

Is it possible to read frames from a video in steps (eg I want to read every fifth frame of a video stream). Currently I'm doing this as a workaround but it's not very effecient.

bool bSuccess int FramesSkipped = 5;  for (int a = 0;  < FramesSkipped; a++)       bSuccess = cap.read(NextFrameBGR); 

Any suggestions so I do not have to loop through the five frames to get the desired frame?

Answer 1

I'm afraid there's not much you can do and it's not just a shortcoming of OpenCV. You see, modern video codecs, are, generally, complex beasts. To get a higher compression rate the encoding of a frame is often dependent on previous and sometimes even successive frames.

So, most of the time you have to decode frames before the desired one even if you don't need them.

There are rather non-trivial tricks to specifically encode a video file, so that it would be cheap to get every Nth frame, but it's not feasible in general case.

That said, you can try the seeking functionality OpenCV provides (see OpenCV Seek Function/Rewind). It may (as well as may not) work faster depending on the circumstances. However, personally, I wouldn't bet on it.

Answer 2

I've had success in Python 3 using a simple counter and setting the capture to that counter's frame, as follows:

import cv2  cap = cv2.VideoCapture('XYZ.avi') # For streams: #   cap = cv2.VideoCapture('rtsp://url.to.stream/media.amqp') # Or e.g. most common ID for webcams: #   cap = cv2.VideoCapture(0) count = 0  while cap.isOpened():     ret, frame = cap.read()      if ret:         cv2.imwrite('frame{:d}.jpg'.format(count), frame)         count += 30 # i.e. at 30 fps, this advances one second         cap.set(cv2.CAP_PROP_POS_FRAMES, count)     else:         cap.release()         break 

I've tried to find a way to make this a little more pythonic using a with statement but I don't believe the CV2 library has been updated for it.